COOLANT RESERVOIR BOILING

I have been having problems with my cooling system in my 94 Lexus GS300. I had my radiator replaced and it was doing fine but then one day after driving on the freeway for about an hour and a half I saw the temp gauge was in the red. I pulled over and the coolant reservoir was full and bubbling. When it cooled down I opened the radiator cap and there was no fluid in the radiator. I filled that up and as long as I refill it every couple of weeks the temp does not go up, but the reservoir keeps boiling. Is this a water pump issue, or have I blown a head gasket? There is no white steam coming from the exhaust, and the coolant stays a clear green color in the reservoir and in the radiator. Also, I have had no problems with performance. What is going on? I had a pressure test done, the mechanic said there was a compression problem in the engine but he wasn't 100% sure on what it was.